1. Field of the Invention
The present invention generally relates to an information reproducing apparatus in a disk playback system, and in particular to an adaptive equalizer for use in a digital data reproducing apparatus reproducing information which was continuously recorded on a disk medium along a data recording guide groove having a periodically wobbling sector format formed on the disk medium. The adaptive equalizer adaptively equalizes each of transmission signals having different signal formats in quality and multiplexed in time basis.
2. Description of the Prior Art
Recently, there has been increasing utilization of various magnetic or optical disks such as CD or DVD having a sector format formed with a recording guide groove periodically wobbling. When information is read out of such a disk, the read-out data signal is supplied to a PLL circuit which generates a clock synchronized with the read-out signal to reproduce digital data synchronized with the clock signal.
In a structure of a sector format formed on a disk, a guide track formed in, e.g., a RAM portion on the disk includes a groove portion and a land portion. The guide track is formed in such a manner that a laser beam spot projected from an optical pick-up head tracks a specified position when information is recorded on the disk. The groove portions and land portions are continuously alternated every one rotation of the disk where the information can be recorded both on the groove and land portions. The guide track is divided into a plurality of sectors and each of the sectors is comprised of an ID region and information recording region. In this example, although the guide track is formed in a spiral format, it may be of a concentric circle format, and also it may be reversible in spiral direction.
In the case where the original data recorded on, for example, an optical disk medium, is reproduced by means of a pickup head, if an optical axis of an object lens of the pickup head is not perpendicular to a surface of the disk but inclined at some angle with respect to the surface of the disk, there may undesirably by generated a warp distortion in a waveform of the reproduced signal output of the optical head. As a method of removing the distortion components included in the reproduction signal waveform, there has been conventionally used an adaptive equalizer utilizing a finite impulse response (referred to as “FIR”, hereinafter) filter.
Specifically, an adaptive equalization is recently carried out in a digital data processing system by previously quantizing the reproduced signal using an A/D converter. Examples of such an adaptive equalizer are disclosed in, e.g., U.S. Pat. No. 5,938,789, U.S. Pat. No. 5,870,372, European Patent Application EP 805448 A2 and Japanese Patent Laid Open Publication 9-320198, where the European Patent Application EP 805448 A2 discloses an adaptive equalizer filter that operates according to a well known least mean square (LMS) algorithm. The LMS algorithm for adaptive equalization is also disclosed in a document “Introduction to Adaptive Filters” by Simon Haykin, published in 1984.
According to the least mean square (LMS) algorithm, the FIR filter coefficient vector of the adaptive equalizer is recursively renewed based on Equation (1) as below:h(n+1)=h(n)+μ·e(n)·u(n)  (1)where h(n) represents a vector of filter coefficients before equalization; h(n+1) represents a vector of filter coefficients after equalization; μ is a programmable gain; e(n) represents a sample error between the filter's actual output and a desired output; and u(n) represents a vector of sample values input to the FIR filter. By this arrangement, the filter coefficients (i.e., frequency and phase response of the filter) are adapted until a minimum sample error is achieved.
Particularly in recent years, the data recording density on the recording medium has been remarkably increased and distortion of the reproduction signal due to inter-code interference of the recorded data on the medium has increased, and also a noise influence in a data transmission path cannot be ignored because of a reduction in amplitude of the reproduction signal. In order to improve the signal reading efficiency with a reduction of bit error rate of recorded or playback codes, a playback data detecting method has been employed to detect an optimal playback data by operating a partial response (referred to as “PR”, hereinafter) equalization of an automatic adaptive equalizer in combination with a Viterbi decoding unit, whereby data stream of transmission signals is monitored before and after a specified time point so as to select the most likely data pattern closest to a desired data pattern from among the monitored data patterns to thereby obtain the optimal coefficients of the FIR filter.
FIG. 13 shows a conventional example of a general read channel for generating binary output data from a reproduction signal. In this construction, the read channel includes an automatic gain controller (referred to as “AGC”, hereinafter) 1 for adjusting an amplitude of the reproduction signal to have a constant amplitude, an analog filter 2 for removing noise components of high frequency band and emphasizing necessary frequency band components of the signal, and an A/D converter 3 for sampling the reproduction signal.
The read channel further includes a digital equalizing filter 4 for adaptively equalizing the discrete sampled data to execute a predetermined PR equalization, and a Viterbi decoder 5 for generating maximum likelihood binary data from the discrete sampled data of the reproduction signal. Furthermore, a D/A converter 6 and a voltage control oscillator (referred to as “VCO”, hereinafter) 7 for synchronization are provided in a feedback loop of the A/D converter 3. By applying the adaptive equalization method to the digital equalizing filter 4, the filter coefficients are recursively renewed in accordance with a waveform distortion included in the reproduction signal to thereby realize a predetermined PR equalization. In particular, when data is reproduced from a disk medium such as CD-ROM, DVD-ROM which was continuously recorded with data signals over the entire circumference of the medium, the adaptive equalizer is continuously operated during all the operation time to recursively renew the filter coefficients. Thus, variation in distortion of the reproduction signal is compensated to thereby enhance reliability of the original digital reproduction data.
However, in a general data recordable disk medium such as CD-RAM, DVD-RAM, a sector format is formed in such a manner that data is recorded in sector units and each sector having a specified length is provided with a string of physical emboss pits for managing user data. In a conventional sector format, an emboss pits string including address information was reduced in recording density to have adequate redundancy so that the address information has higher reliability than the user data. However, in recent years, in order to increase a formatting efficiency, there has been put to practical use a sector format such that the emboss pits for recording address information have a recording density as high as that for the user data. For example, a sector format of DVD-RAM is comprised of an emboss pits string called a header field, a mirror field and a user data recording field having a data length of 2048 bytes.
FIG. 14 shows an example of a header field layout having a segment far shorter in data length than that of the data recording field. In this construction, the header field is comprised of two pairs of header field regions, the two pairs being shifted toward an inner or outer peripheral side by a distance of ½ track pitch. The width of the pit signal corresponding to the header region is made generally equal to the width of a groove portion or land portion in the user data recording field.
As shown in FIG. 14, the groove portion and land portion are periodically wobbling in a zigzag or sine wave format in a direction perpendicular to the tracing direction of the track. This sine wave wobble indicates that, when data is recorded on the user data recording region in the RAM portion by operating a disk drive, the period of the wobble is detected to generate a clock signal in synchronization with the detected period so that the data is recorded in synchronization with the clock signal. The wobble appears generally all around the groove and land portions. The laser beams always scan along the center line portion of the track in the data recording field. On the other hand, the laser beams scan the emboss pits region of the header fields in an off-track condition, and the resultant reproduced signal is intermittent in this sector format type disk. Accordingly, if a continuous adaptive equalization is carried out on the intermittent reproduction signal over the header fields, the resultant equalization is erroneous, and a desired PR equalization cannot be obtained, which deteriorates the reliability of the reproduction data.
Moreover, because the laser beams scan the emboss pits region of the header fields in an off-track condition due to the zigzag or sine wave wobble, the distortion components included in the reproduction signal are different between the header field and the user data recording field. Therefore, the optimal equalization cannot be executed for each of the fields by the continuous equalization and the reliability of the detected address information is deteriorated to be a problem.
Moreover, because the header field is much shorter in physical data length than the recording field, there is not secured an adequate time for learning an adaptive equalization within one header field, and therefore this may deteriorate the reliability of the detected address information.